Oak - A Question.

[Rick Dipper]

Knowledgable People

I am making an Oak Truss over the holiday period, I have a worry

I am making the King Post, the bit that goes up the center of the triangle. Before I started I knew there was a hole in the oak, from a branch that was sawn off/died many yesrs before the tree was cut down. I have now found that inside the timber, where this knot was, that timber is very black and wet, with some white flecks in it.

I am in two minds here

1) I want to use this timber, to save the 2+ days work I have done so far

2) I want to get a new bit, cause I don't want my house falling down. The neighbor who supplied the timber will swap it for free.

I have read, that knots do not significantly effect the strength of wood, when the timber is under compression down the grain.

The piece is 50% bigger than the engineer designed, so I have some "play"

Does anybody have any usefull knowledge or reading I that could assist me ?

Thanks Rick

[Anna Kettle]

The Carpenters Fellowship is the UK association of people who do this for a living and on their webpage

a discussion forum. Ask a question, they're a nice bunch

Anna

~~ Anna Kettle, Suffolk, England |""""| ~ Lime plaster repairs / ^^ \ // Freehand modelling in lime: overmantels, pargeting etc |____|

01359 230642

[Anna Kettle]

Hm on second thoughts. The equivalent plasterers discussion forum is used very little, most replies being made by some guy in France with somewhat odd ideas. Play it by ear. Not many builders are internet hobbyists

Anna

~~ Anna Kettle, Suffolk, England |""""| ~ Lime plaster repairs / ^^ \ // Freehand modelling in lime: overmantels, pargeting etc |____|

01359 230642

[Rick Dipper]

Thanks Anna, I took a look, it looked very quiet. I'm gooa start again, the post holding up the roof should be right, just so I can sleep at night.

Its a strange world, the Internet is my job, building is my hobby..

Rick

[The Natural Philosopher]

"Understanding wood' by Hoadley (sp?), is THE book to get.

But I think you will be fine actually. A timber column is massively strong under compression, unless the Euler instability is reached, and its very unlikley that a knot would weaken it enough for that to happen.

The instablity reduces dramatically for lager corss sectional area, and you have plenty there by the sound of it.

[Rick]

Thank you sir, a copy is on its way to me from Amazon .........

Rick

[Mike]

As a major part of my roof seems to be supported on such a column about 4*4 of two hundred year old oak, any chance you could give a briefer description of what Euler instability is than those Google throws up ?

[Derek *]

In fact I think you could really call it buckling under compression but that would be just vulgar. ;-)

Page here is clearer than most and also explains how you could make your column stronger.

point is it's a system where the failure is driven by positive feedback. Once it starts it goes!

Think of a stack of plates on top of a long whippy pole. If it's perfectly vertical and unloaded it'll be OK but keep adding plates and eventually there will come a point when it will belly out in the middle and fail catastrophically.

DG

[Roger]

The message from Derek * contains these words:

Might be more correct. My limited understanding of Euler is that it applies to slender columns. Dipping into an ancient textbook gives a mind boggling formula for the Euler buckling stress (the mind boggling bit is the "end-fixity coefficient") and the Johnson parabolic equation for "columns whose lengths are less than that necessary for the Euler theory to apply".

I am not sure how much such theory is directly applicable to fibrous materials such as wood. Drilling a large hole into the outer surface of a slender metal strut at anywhere near half height would seriously weaken an already potentially unstable structure.

[The Natural Philosopher]

Its the point at which a column is long enough and floppy enough to collapse from bending and buckling, rather than from straight compression.

I.e. consider a thin piece of timber: If you make a pole out of it and put weight on, eventually it will snap sideways.

If however you take an inch of it, it will probably support a ton at least before crushing.

Euler did some fancy maths and showed that for any tiny flexure (infinitesimal) in a slender column, there came a point at which the bending would be unstable, and increase at a certain loading.

This limits the 'strength' of slender columns not to the compressive strength of the material, but to its bending resistance.

To reduce insatbility you e/g. use hollow columns (since you want stiffness) or simply large cross sectional area or braces.

With a massive king post I would expect it to be able to do many many tons before buckling. But an engineer who specialies in these things would know more.

My point being however, that the way the flaws in the wood affect bending, is far more crucial in a post than the actual loss of compressive strength.

I believe the book I recommended has some tables for amounts of flaws versus changes in wood physical properties.

[The Natural Philosopher]

Eulers criterion applies to any material.

Its a point at which the compressive strength becomes irrelevant, and buckling resistance becomes the failure mode.

One normally forgets the 'end fixity coefficient' as unless your post goes down 12 feet into concrete its not really an issue.

I didn't want to start a huge discussion on it - merely to point out that 'compressive strength' is almost certainly a red herring in this case.

Wood structural elements fail almost always in bending, and if they don't they are massiveley over engineered. Wood JOINTS fail in other ways, and Hoadleys book gives all the detail one could wish for.

[Andy Dingley]

Not for framing it isn't. Hoadley assumes that you're going to leave the finished product indoors. If you're doing big structural stuff or green framing, then the US forestry dept. timber products handbook is better (free as PDFs, or you can buy a nice printed and bound copy from Lee Valley). However Hoadley _is_ an excellent book and well worth the read.

I suggest you talk to a timber framer. They're largely penniless hippies who under-charge ridiculously and will do this work more easily than you can (oak isn't the easiest stuff to work) and can supply decent grade materials at good prices (hardwoods are chepar in the UK than you'd believe, you just have to buy them in the right places). There is no way I would have used oak branchwood like this for this type of structure. The work is too hard relatively to the risk of it causing problems. Of course you _could_ do it, but the extra trouble it causes just isn't worth it unless you're doing something deliberate, like cruck framing.

If you're near Bristol, drop me a line for a recommendation.

Euler failures won't apply to timber (in any sensible scenario), because of the relative values of strength/stiffness and the strengths of timber and its joints. A timber post will pop out of its fixings long before the post itself is going to fail in this manner.